Retroreflective sheeting is used in various applications including, for example, in road signs, pavement markers, license plates and clothing. Many of these applications require the sheeting to have an eye pleasing or cosmetic appearance.
One useful type of retroreflective sheeting is cube corner retroreflective sheeting. Cube corner retroreflective sheeting typically includes a sheet having a generally planar front surface and an array of cube corner reflecting elements protruding from the back side. In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of the intended observers. In this orientation, light incident to the front surface enters the sheet, passes throughout the body of the sheet to be internally reflected by the faces of the cube corner reflecting elements so as to exit the front surface in a direction substantially toward the light source, i.e., retroreflection.
The manufacture of retroreflective cube corner element arrays is typically accomplished by employing molds or tools which may be multigenerational copies of master tools. Master tools are primarily made by known techniques such as pin bundling, laminae assembly and direct machining Tools manufactured by pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner reflective element. Tools manufactured by laminae assembly are made by assembling together individual lamina or blades which each have a side portion shaped with features of cube corner reflective elements. The direct machining technique involves cutting away portions of a substrate to create a pattern of grooves that intersect to form structures including cube corner elements. This grooved substrate is typically used as a master from which a series of impressions, replicas or molds may be formed. These are typically then used as molds for retroreflective sheeting.
Once the mold or tool is made, retroreflective sheeting can then be made by any suitable method including, for example, either by thermally embossing a plastic sheet with the tool substrate to form a molded surface or by subsequently depositing a crosslinkable, partially polymerized resin on a mold to be microreplicated, which is then typically exposed to radiation such as, for example, actinic light or heat, to solidify the resin.
Such manufacturing processes are typically continuous processes. For continuous manufacturing of retroreflective sheeting, a tool is typically formed from discrete tooling elements, which are generally flat in nature, by joining them together with one or more welding lines across their widths. The first discrete tooling element (the master) is usually created as described above and then multiple copies comprising multigenerational copies are made through electroplating. These copies are then welded together into the form of a cylinder, belt or sleeve with one or more welding lines across the width of the cylinder, belt or sleeve.
When the retroreflective sheeting is made on the tool, resin flows into the weld lines and causes seam lines to be replicated in the resulting sheeting. The seam lines can typically be observed across the width of the retroreflective sheeting. Because these seam lines reduce the cosmetics of the sheeting, attempts have made narrow them or to eliminate them. For example, U.S. Pat. Nos. 5,643,400 and 5,558,740 (both to Bernard et al.) describe an apparatus and a method for producing retroreflective sheeting wherein at least two mold surfaces are used to generate two prism arrays which are overlapped at a leading and/or a trailing edge of each array. U.S. Pat. No. 6,709,258 (Paulson et al.) describes molds for retroreflective sheeting made by welding the opposing ends of the back side of a substantially planar tooling together to form a cylindrical shape.